1. Field of the Invention
The present invention relates to a high-density energy beam welding process, and-more particularly to a method of welding abutting workpieces with a high-density energy beam and a structure welded by a high-density energy beam.
2. Description of the Prior Art
High-density energy beam welding processes include an electron beam welding process, a laser beam welding process, etc.
One known laser beam welding process is disclosed in Japanese laid-open patent publication No. 3-230883.
When metallic members of aluminum or aluminum alloy are welded to each other, a molten pool is formed around a keyhole between the metallic members. The molten pool has an inner region which surrounds the keyhole and in which the molten metal is highly actively agitated, and an outer region which is disposed around the inner region and subjected to less active agitation. When the molten pool grows into a zone where the confronting surfaces of the workpieces contact with each other, aluminum itself is melted as it has a low melting point, but an unmeltable metal oxide film of Al.sub.2 O.sub.3 having a high melting point is not melted. The portion of the Al.sub.2 O.sub.3 film which is positioned in the inner region of the molten pool is broken and dispersed by the highly active agitation. However, the portion of the Al.sub.2 O.sub.3 film which is positioned in the outer region of the molten pool is not destroyed, but remains in the outer region as joined to an unmelted zone of the workpieces. The residual Al.sub.2 O.sub.3 film is responsible for forming a cavity in the united region which is created when the molten metal is solidified. The cavity that is present in the joint results in a large reduction in the mechanical strength of the welded joint.